3.1a Newton's Laws and Forces

Newton's First Law - The Law of Inertia

• An object will remain at a state of rest or uniform velocity unless acted upon by an external or unbalanced force

• A sprinter will remain in the starting blocks until a force acts upon it to overcome its inertia

• The greater the mass of the sprinter, the greater the force needed to accelerate out the blocks

Newton's Second Law - The Law of Acceleration

• A body's rate of change in motion is proportional to the size of the force applied, and acts in the same direction of the force applied

• A sprinter in the starting blocks will accelerate out of the blocks at a greater rate if they push off with a greater force (due to the greater change of momentum)

Newton's Third Law - The Law of Reaction

• For every action there is an equal and opposite reaction force

• The sprinter will apply the downwards and backwards force in the blocks, and the blocks will provide an equal and opposite upwards and forwards reaction force

Internal force

• contraction of the skeletal muscle - the quadriceps will contract to cause the knee to extend

External force

• contraction of the skeletal muscle - the quadriceps will contract to cause the knee to extend

Effects of forces - create motion

• a ball will remain at rest until a force is applied

• a sprinter will remain still until a force is applied to cause them to move

Effects of forces -accelerate a body

• the greater the force of the foot kicking the ball, the greater the acceleration of the ball

• the greater the force from a javelin thrower's shoulder, the greater the acceleration of the javelin

Effects of forces -decelerate a body

• as a ball moves through the air, air resistance will act the opposite way and slow it

• air resistance will slow a javelin and decrease the distance it travels

Effects of forces - change the direction of a body

• when a goalkeeper's hand touches the ball, the force will cause the direction of the ball to change

Effects of forces - change the shape of a body

• a ball touches a net, and the force from the ball will change the shape of the net

Net force

• the sum of all forces acting on a body - resultant force

• net force 0 = no change in motion - balanced forces

• if net force is present, the forces are unbalanced so there is a change in motion

  • two forces opposite in direction and unequal in size

Weight

• gravitational pull that the earth exerts on a body

• Weight = mass x acceleration due to gravity

• measured in Newtons

Reaction

• the equal and opposite force exerted in a body in response to the action force placed upon it

• result of Newton’s third law and is always present when two bodies are in contact

• measured in Newtons

Friction

• opposes the motion of two surfaces in contact

• measured in Newtons

Factors affecting friction - ground surface roughness

Athletes run on rough, rubberised tracks

Factors affecting friction - contact surface roughness

Athletes wear spike shoes

Factors affecting friction - temperature

F1 drivers have a warm up lap

Factors affecting friction - size of reaction force

Shot putters have a higher mass and so a higher reaction force

Factors affecting AR - velocity

increased velocity of a cyclist increases the air resistance opposing their motion

Factors affecting AR - shape

cyclists wear a helmet in an aerofoil shape to streamline them

Factors affecting AR - frontal cross sectional area

the low crouched position of a downhill skier reduces air resistance

Factors affecting AR - smoothness of surface

increased smoothness from lycra suit reduces air resistance

Velocity

Velocity = displacement/ time taken

• velocity = m/s

• displacement = m

• time = s

Momentum

Momentum = mass X velocity

• momentum = kgm/s

• mass = kg

• velocity = m/s

Acceleration

Acceleration = (final velocity - initial velocity)/ time taken

• acceleration = m/s/s

• change in velocity = m/s

• time = s

Force

Force = mass X acceleration

• force = N

• mass = kg

• acceleration = m/s/s

Free body diagrams

Weight = from CoM vertically downwards

Reaction = from point of contact directly upwards

Friction = from point of contact extending horizontally in the direction of motion

Air Resistance = from CoM horizontally in the direction opposing motion